1. Field of the Invention
This invention relates to transponder systems for communicating with a tire vehicle, and more particularly to systems which use a diode circuit to produce an information-bearing frequency shift in an interrogation signal.
2. Description of the Related Art
Various systems have been devised for automatically communicating with a vehicle tire to obtain status information on the tire, such as its air pressure, or to identify the tire. Current information on a tire's pressure is particularly important for new "run flat" tires, in which the tire has an inner core that allows the vehicle to continue running when the outer tire has gone flat. The driver may not be aware of the run flat condition, since the bumps normally produced by a conventional flat tire are avoided by the run flat feature. If the driver is not informed of the flat tire, the tire can be permanently damaged by driving too far before the flat is repaired.
Tire identification is useful for tracing stolen tires, warranty work and similar applications. Both pressure sensing and tire identification are combined in certain truck depots, in which the truck tires pass over a tire pressure sensor as the truck leaves the depot, and a tire identification is automatically provided that tells which truck has the low tire pressure.
Prior tire air pressure sensors have employed full two-way radio telemetry systems, in which the absolute tire pressure is transmitted to a receiver which is located either on the vehicle, or at a fixed location such as a truck depot. The radio equipment is typically mounted to the valve stem inside the tire, and uses a small dipole antenna to transmit a very high frequency RF signal, typically 900 MHz. The equipment is subject to being damaged by excessive tire wear or by the tire going flat, requires modification of the tire to be installed, and is relatively expensive. Furthermore, 900 MHz is not within an internationally recognized frequency range for this type of transmission.
Automatic tire identification is normally accomplished by gluing an identification "tag", consisting of an integrated circuit (IC) chip together with a small antenna coil, to the inside of the tire. The tag transmits an identification code which uniquely identifies the tire. However, the tag can suffer from the extreme temperatures and mechanical stresses which tires normally experience. Also, a reader attached to the vehicle can read a signal from a tire on an adjacent vehicle when the identification encoding is less than optimum, as is sometimes the case.